Tubular articles are used in different industrial applications. In the automotive industry, the cars manufactured may, for example, be provided with tubular side beams and also with bodies of so-called space frame construction. It would seem convenient to manufacture these types of tubular articles by means of expansion forming.
In expansion forming, an expansion forming tool is used. The tool comprises a cavity having the shape intended for a finished article. The tool usually comprises two milled-out tool halves made of steel, the recess in the tool halves forming the cavity. An initial blank having a hollow space is pre-bent to fit the cavity and is thus arranged between the tool halves, which are then closed against one another. One end of the blank enclosed by the tool is sealed and a pressure medium is supplied at the other end, for example by means of a tubular lance provided with a seal, which is inserted in the pre-calibrated tubular mouth of the blank.
The expansion forming is achieved, for example, by means of hydroforming. This means that a pressure medium, such as oil or another liquid, is pumped into the hollow space of the blank. The forming pressure of the pressure medium pumped in is usually in the range of 1000-6000 bar, but both lower and higher pressures can be used. The choice of forming pressure depends on different parameters, such as the material, shape and desired tolerances of the article.
Conventionally, a press platen is used which covers the upper side and/or the under side of the tool and which is applied to the tool by means of the closing force of the press. The forming pressure generated in the tool by means of the pressure medium will produce an opening force aiming at separating the tool halves. Thus, the opening force is generated by the forming pressure of the blank multiplied by the area of the exposed blank transversely to the closing force. In the case of large blanks having large areas of exposure and high forming pressures, large opening forces are generated. When combined with large platen areas, this results in large tolerances for the formed product. It is difficult to keep the downward deflections in large press platens within the desired limits. When great forces are involved (more than 10,000 tons, i.e. about 100 MN), building plants in conventional manner becomes financially doubtful. The forming tools and, possibly, the press platens would need to be very thick. Examples of possible deformations are illustrated in the accompanying FIGS. 2a-b and 3a-b. 
WO 00/00309 A1 discloses a device and a method for expansion forming. In this specification, a pressure cell provided with a diaphragm is used, which exerts a pressure both on a pressure intensifier to generate an internal forming pressure in a tool and on the tool itself. The pressure cell provided with diaphragm contributes to the drawbacks mentioned above being alleviated, since the force used to close the tool is transmitted by the diaphragm through a pressure medium and is evenly distributed over the upper side of the tool. Although such an arrangement has certain advantages compared with conventional press platens, it is still not enough to prevent deformations of the tool or avoid unevenly distributed loads at very high pressures. Consequently, more rigid tools would be required also in this case, which is a problem because it means that a thicker tool is needed to obtain the desired rigidity.
Accordingly, it is desirable to be able to form large articles at a reasonable cost.